Hi guys, thank you for all the lovely enthusiasm about my idea of a Nail Polish Science series! It’s given me loads of motivation to crack on with writing, so here’s the first full post: the chemistry of colour.
I am going to pitch my explanation on the assumption that most people don't remember much high-school science – zero maths, minimal jargon, and clear explanations of the very few technical terms needed (apologies to people who do have the background, but hopefully it will still be interesting!). So without further ado, let's dive in!
First, a quick word about light
As we all know very well, light is the key to colour. The full spectrum of light is divided into a bunch of categories based on energy, including UV, IR and visible light, visible obviously being the part that we can see. We can choose to describe light as a bundle of waves, or as a bunch of particles called photons, whatever is more convenient for what we’re trying to explain. Here, I will mostly talk about photons, and we can talk about single photons of different energies e.g. a photon of visible light will be lower-energy than a photon of UV, a photon of red will be lower-energy than a photon of blue.
As I’m sure we all know (but just in case) white light is made up of the entire visible section of the light spectrum added together (side note: this section has the highest intensity in sunlight, which is presumably why we evolved to make use of it). Most of us have three sets of cone cells in our eyes: red, green and blue, dividing that visible part of light up into three. When they fire at equal rates, we see white (or grey, if they’re equal but less intense). When they fire at different rates, our brains interpret that as colour e.g. when the blue cones are firing more intensely than the other two, we see that as blue.
This is what a pigment does: white light falling on a blue pigment would have all of its red and green photons ‘stolen’ and all the blue photons reflected back (also why a blue pigment looks black under a red light – no blue photons to get reflected!). To explain that, we need to understand what the electrons in the pigments are doing.
Intro – chemistry nerd time
TL;DR: electrons have specific paths (called orbitals) they’re allowed to travel around an atom or molecule, and they are stuck in their path unless they get an energy boost to jump to another orbital. The energy boost has to be exactly right, or else the jump can’t happen.
More detail: Time for a little primer on electrons, energy levels and orbitals, an extremely fundamental concept in chemistry that is vital to understanding how most colour happens. All atoms and molecules have orbitals where electrons ‘sit’ and that is the only way an electron can part of an atom/molecule. Orbitals are essentially a way of describing where an electron is allowed to hang out/the path it’s allowed to take (like driving along a road, instead of right through the wall of a building). They all overlap each other in physical space, like fuzzy blobs phasing in and out through each other, with all the electrons constantly whizzing past each other.
You might remember drawing diagrams of the electrons around an atom as though it were a solar system, or dot-and-cross diagrams of molecular bonds in school – that’s a very simplified representation of this concept. The relevant extension here is that all molecules have a bunch of these orbitals, which are made by the mingling of atomic orbitals, and there are always filled and unfilled ones present.
An electron’s energy in an orbital is always lower than a ‘free’ electron in the vacuum (and we always compare the orbital energy to that free state) as it's stabilised by attraction to the nuclei (if it wasn’t lower, the electron would just run away to the vacuum again). For a specific orbital, this energy is extremely well defined e.g. all oxygen molecules have exactly the same energy for an electron in their highest-energy filled orbital.
The way I make sense of this to myself is by saying the highest-energy/least-stable level is like walking a tightrope, while a lower-energy/more-stable one is walking on solid ground, and even lower energy, sitting in a chair or lying in bed. Electrons will always ‘want’ to be in the lowest-energy, most-stable state available (relatable – I know I prefer scrolling Tumblr in bed to doing my grocery shopping LOL). So if they do get a boost up to a higher state, they (usually) pretty quickly release the energy and fall right back down again, re-releasing the absorbed energy and resetting their jump-up-ability.
So what about the polish colours?
TL;DR: the energy for an electron’s jump up to a higher-energy orbital can be from a single photon, which gets absorbed; in a pigment molecule, that photon will be in the visible range. When the electron falls back to its original path, it is emitted as heat energy instead of another photon. So all the photons of that colour get removed from the spectrum, and we get the rest of the photons reflected back, because they are the wrong energy to interact. Since it’s no longer the full spectrum of visible light, but only some parts of it, we see it as a colour.
More detail: One way in which an electron can jump up to a higher-energy orbital/path is by absorbing a single photon, whose energy corresponds exactly to the difference in energy between the two orbitals. It can’t be two photons that add to give the required energy, or even a slightly too-high-energy photon with a little energy left over: it has to be one photon with exactly the right energy. It's like landing a rover on the Moon: overshooting even slightly makes the mission as pointless as undershooting. Here’s a helpful diagram: the horizontal lines represent orbitals of increasing energy, while the arrows represent jumps that can happen between them (ignore the Greek letters and stuff, we needn’t get into that).
For a pigment molecule, that photon is gonna be part of the visible spectrum of light. Importantly, that means that this colour of light is absorbed, while the rest of the photons are reflected back, because they are the wrong energy to interact with the molecule in any way. So a green pigment is actually absorbing red and blue light, leaving the green to reflect back into your eyes (worth noting that the electron might also go into the second-lowest unfilled state, so if the photon responsible for that also is in the visible region, two photon colours are absorbed by the pigment. On the diagram, these are the leftmost and rightmost arrows respectively. In our Moon-landing analogy, if we gave the same rover a bunch more energy, it could reach Mars or even Jupiter (or something, I’m not an astrophysicist LOL)).
This is called ‘subtractive colour’ and it’s how pretty much all pigments and dyes work. A white pigment will reflect back all the light that falls on it, while a black one will absorb all of it (this is why black objects heat up faster! AKA my hair on a sunny day, you could fry an egg on it). Computer screens are different because they produce the light and beam it directly into your eyes, which is ‘additive colour’.
Your questions answered!
In the previous post, people asked a few related questions that I thought would be good to cross-reference, plus I can go into more detail on some of them after having explained the basics. I have to say there were some really great questions that made me think about things more deeply, and ultimately helped me do a better job of explaining this topic, so thank you for that! I also added a couple more that I thought might come up, or that I wanted to talk about anyway but flowed better here.
Q: What about jellies?
Literally the exact same thing, just more dilute pigment. This is why I refuse to buy them LOL, I can get a bottle of clear polish for £4 and mix my own, instead of paying £15 for a Cirque *cries in non-US stockist markups*
Q: I’ve heard cyan, specifically, is a hard pigment to make. Why is that?
A cyan pigment would need to absorb only red light, because cyan is green+blue light. The problem with this is that red is the lowest-energy part of visible light. That means that to absorb it, a pigment needs to have a relatively small jump between its highest filled and lowest unfilled orbitals, corresponding to a low-energy red photon. BUT remember, we said that a jump up to the next highest unfilled orbital, using a slightly higher-energy photon, is also possible – that will very probably correspond to a green or blue photon! So it’s gonna be really hard to make a single pigment that only absorbs red and not Also green or blue.
The way it's probably done is by mixing a green pigment (where you overshoot green on the second jump and absorb red+blue and reflect just green) with a pure blue one (which absorbs red+green and reflects just blue) but that might introduce a bit of murkiness, I don’t know. I’d imagine a pure pigment is always going to give you a purer colour, because the subtractive colouring doesn’t overlap or leave sections out. Maybe this is also why it’s so hard to find turquoise green polishes that are really bright but with no white undertones, which is my constant woe because I love that shade so much *cackles over precious hoarded bottles from five years ago, when a random UK pharmacy brand had a really nice one*
Q: What about fluorescents/glow-in-the-dark?
So earlier, I said electrons usually fall down quickly from the excited state. In some specific molecules, they find it much harder to return, because of the way they are now sitting in their orbitals (to slightly misapply our earlier analogy, I sadly can’t teleport from the grocer’s to my deskchair, but tumbling from chair to bed is way easier). But the electrons do eventually fall back down, over a timescale of minutes/hours rather than billionths of a second. In this case, they do release the energy as a visible photon rather than random heat energy. That’s your glow-in-the-dark effect, because you have enough electrons staying in the higher-energy state for a while after you take away the main light source, and a more gradual return to the original orbital and corresponding photon emission.
Q: What about thermals/solars?
It’s a similar basic principle to these pigments, with added complications regarding the ‘switch’ between colour states. I wrote a long comment about it here, and am probably going to make it into its own post, because it’s tricky to explain well in a single paragraph and this post is more than long enough already LOL (I plan to include more technical detail in the upcoming post than I did in the comment, on a similar level to this post).
Q: What about multichromes/shifties/aurora/iridescents/Unicorn Pee? What about holos?
Those are both completely different effects to the solid-coloured pigments, and are much better explained through a physics lens! Stay tuned, more coming soon on this 🙂
Q: You keep banging on about molecules. What kind of molecules or substances are we talking – minerals, oils or what?
Typically, metal ions are particularly good at having jumps that correspond to visible photons, and those give minerals their colours. Sadly those are often very very toxic, so we typically fake the effect with organic (carbon-based) molecules instead (any time you see a ‘Lake’ pigment, it means it’s organic and not mineral-based). This also gives us a lot more control over exactly what the energy jump is, by tweaking the exact structure of the molecule, which means we can have synthetic dyes and pigments in colours that are a lot harder to make naturally. Some of them are found in nature, like indigo for blue denim, but many modern ones are synthesised.
Q: So is that what causes staining/yellowing of the nails?
Not really. The difference between dyes and pigments is that dyes chemically bind to the thing you’re colouring, while pigments sit on the surface, but the chemistry of how the colour is produced is the same. A chemical that dyes one surface may not dye another, it depends on whether the chemical reaction between it and the surface can happen or not, or you can make it happen by tweaking the part that reacts with the surface. In nail polish you want pigments, because something capable of dyeing the nail will, of course, cause staining (I’ve definitely had this from really cheap polishes, though!) However, a really saturated pigment might still wriggle into the top layers of the nail without chemically bonding, which will also cause staining (and is why I never, ever skip the base coat). The yellowing we all get from constant polish abuse is because the nitrocellulose in pretty much all lacquer/non-gel polishes (see my earlier post for more on this) reacts with the nail surface, which is unfortunately unavoidable unless you find a nitrocellulose-free base coat, maybe.
Q: What about single-colour shimmers, glowies and general glitters?
Shimmers, pearls, glowies, microglitters and metallics are all just coloured particles that are smooth and reflect light well, but are too finely milled to see the individual particle with the naked eye (unlike, say, the Holo Taco Unicorn Skins where you can see every individual flakie and how it reflects light). Glowy polishes have a jelly base that may contrast with the shimmer particles, producing that pretty contrasting flash of colour when the light hits the shimmer particles.
For pearls, shimmers and glowies, it’s all about letting as much light through as possible, which is why they’re typically in a clear or jelly base, and my guess is that the particles are translucent to allow the lower layers to shine through. Metallics and opaque microglitters are a little different – they have opaque foil-backed particles that act as tiny mirrors. Reflectives, it seems, are actually tiny smooth glass beads, so different yet again!
The difference between these effects and cremes/jellies: the creme has individual pigment molecules floating around in solvent, rather than larger bits of plastic or mica or whatever. So the very chunkiest glitters, that you can see with the naked eye/have to fish in the bottle for/poke around with a cocktail stick to make them look nice, are a few millimetres across. Then you have a sliding scale of glitter sizes down to the very finest glitters, which will probably be around a few microns, or thousandths of a millimetre. Then you get a huge plummet in size to individual pigment molecules – they’re going to be a few nanometres, or thousandths of a micron, and coloured shimmer/glitter particles will have a bunch of them inside the plastic.
Q: So glitter particle size has a big effect on the finish?
A very significant one! The tiniest glitters create a smooth metallic effect because your eye can’t pick out the individual sparkles. This probably also means they can be more densely packed because they interfere less with the liquidity of the polish, which would explain why I have a lot of one-coater metallics and fine shimmers, but find that the chunkier ones need to be built up. The size order therefore goes something like this: individual pigment molecules <<< pearls/metallics < shimmer/microglitter < fine glitter/tiny flakies → chunky glitter and bigger flakies that large enough to be easily seen by the naked eye.
There’s a pretty big difference in how the individual pigment molecules behave in the liquid polish, versus even the tiniest glitters. This explains why pigments don’t need a suspension base but everything else does: it’s a fight between gravity, which ‘wants’ to pull the solid glitter particles down to the bottom, versus diffusion, which ‘wants’ the mixture to be as even as possible. So the glitter particles need a thicker base that helps thwart gravity (and even then I’m sure we’ve all noticed that chunky glitters do tend to settle at the bottom). But in cremes/jellies, which have individual pigment molecules, diffusion typically ‘wins’ even in a normal clear base (I’ve successfully made jellies using just clear polish that wasn’t formulated as a base, but it ended up a sloppy mess when I tried to make a shimmer topper by diluted a pearly polish).
Sources:
Various uni lecturers who I will not cite individually, because I don’t want to get doxxed for where I went to uni LOL. I took all the maths out though, you’re welcome 😛
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Map%3A_Physical_Chemistry_for_the_Biosciences_(Chang)/14%3A_Spectroscopy/14.07%3A_Fluorescence_and_Phosphorescence/14%3A_Spectroscopy/14.07%3A_Fluorescence_and_Phosphorescence)
https://www.sciencedirect.com/science/article/pii/B9780125551601500228 (diagram used earlier). Full citation: DONALD J. PIETRZYK, CLYDE W. FRANK, Chapter Eighteen – Qualitative Analysis: Ultraviolet, Visible, and Infrared, Analytical Chemistry, Academic Press, 1979, Pages 410-424, ISBN 9780125551601, https://doi.org/10.1016/B978-0-12-555160-1.50022-8.
www.discoverbioglitter.com/bioglitter_physics_of_light/
https://www.nailsasjewels.co.uk/shop/Purple-p522191354
https://glowtec.co.uk/reflective-powder/
Upcoming topics:
- Thermals/solars
- Multichromes/shifties/iridescents/aurora/etc
- Holo effects
- Miscellaneous formula-related stuff: a little more on curing and gel vs regular lacquer. Why polish and water don’t mix/why humidity causes bubbling. QDTCs and quick-dry drops; crackle polishes. (Hopefully, if I can get my head around it myself) why PVB in base coats causes peeling for some people. Mayyyybe a bit on fluid art if, again, I find enough material on it.
I would love further questions/topic suggestions! OR, if you know better than I did about something I’ve said, I would also welcome corrections 🙂 (with the caveat that I’ve obviously deliberately simplified a lot of complex concepts, which unfortunately does introduce some level of inaccuracy/overgeneralisation). I'm afraid I'm going to have to go back on my earlier promise of tagging anyone interested, because I just do not have the time now, but I hope all the interested people find it anyway!
Finally, thanks very much to u/happierthanuare, u/cation587 and u/Various_Platypus9222 for proofreading and fantastic feedback!
by apricotgloss